Up to now, when communication is conducted with radio connection in a multi-cell system (cellular system), a radio terminal communicates with a radio base station having a matched service set identifier (SSID). Then, when the radio terminal travels while calling the radio base station, electric waves become weaker with increasing distance from a center of a cell (wireless area), and a signal strength and a power level are changed. In this situation, when electric waves from another radio base station of an adjacent cell become more intense, call can be continued if the communication is switched to communication with this radio base station. This is called “handover”.
In order that the radio terminal accesses to a new radio base station, the radio terminal must search identifiers of a large number of radio base stations corresponding to connection candidates or having connection histories in order, and acquire a connectable radio base station. Further, radio base station cannot conduct communication through a network unless the radio base station registers information on itself and the subject radio terminal in a registration server of an IP network.
FIG. 32 is an illustrative diagram illustrating a conventional handover of a radio terminal. An IP network (network communicatable in IP) is connected with a management device ME having the registration server therein. Also, the management device ME is connected with radio base stations (hereinafter referred to as “radio cell stations”) CS1, CS2, CS3, CS4, and CS5 under its control, which each radiate electric waves to form a cell, and form a communication area as a whole. Radio terminals (hereinafter referred to as “radio personal stations”) PSs include a PHS (personal handy-phone system) terminal, a DECT (digital enhanced cordless telecommunication) terminal, other cellular phones, and a radio LAN (local area network). Anyway, each of the personal stations conducts a digital radio communication with any radio cell station. A voice data communication between each of the radio personal stations and a partner terminal conforms to an IP call control protocol such as an SIP (session initiation protocol) or H.323, and is conducted with the use of RTP/RTCP.
In order that the radio personal station PS connected to the radio cell station CS1 is connected to an IP terminal of an intended party through the IP network, the management server such as an SIP server needs to recognize which radio cell station CS is connected to the radio personal station PS at present. For that reason, the radio cell station CS1 transmits a REGISTER (position registration request) to the registration server of the SIP server, and registers identification information such as its own IP address and an identifier (telephone number) of the radio personal station PS in the registration server.
Accordingly, when the radio personal station PS travels from a cell of the radio cell station CS1 to another cell (radio cell station CS3) during a call, the radio personal station PS cannot continue the call unless the radio connection is switched from CS1 to CS3. For that reason, handover is conducted, and in conducting the handover, the radio cell station CS3 transmits the REGISTER (position registration request) to the registration sever, and registers the position of a new radio cell station CS3. As a result, the radio cell station CS3 allows the registration server to recognize disconnection and reconnection. In this situation, unless smooth handover is conducted, the call is disconnected to deteriorate a call quality.
Likewise, thereafter, every time the radio personal station PS travels from a cell of the radio cell station CS3 to a cell of the radio cell station CS4, from the cell of the radio cell station CS4 to a cell of the radio cell station CS5, and from the cell of the radio cell station CS 5 to the cell of the radio cell station CS2, the radio cell stations CS4, CS5, and CS2 transmit REGISTER, and repeat the position registration in the registration server. The call cannot be continued unless the position registration is conducted. However, it takes time to conduct the handover, and when reconnection fails, the radio personal station PS must again search the radio cell station CS for reconnection.
Up to now, a technology has been proposed in which handover is conducted when a user travels from an area of one MGW (media gateway) currently connected to his cellular phone to an area of another MGW while calling by the cellular phone through an IMS (IP multimedia subsystem) (refer to JP-A 2009-177620).
When a termination (TDM/1) of a first communication system is calling a termination (RTP/1) of a second communication system within one MGW 1, a mobile station travels to an area falling under the control of another MGW 2. In this state, a context of a termination (RTP/11) of the first communication system is added to a context of the MGW 1 that is a handover source. Further, a context for connecting a termination (TDM/2) of the first communication system and a termination (RTP/2) of the second communication system is added to a context within the MGW 2. It is assumed that the TDM/1 and the RTP/11 are one-way stream, and downstream with respect to the MGW 2.
In a handover switching state, the RTP/1 and the TDM/1 are changed to one-way stream, and the RTP/1 and the RTP/11 are changed to two-way stream to conduct the switching of the streams of a source and a destination. Finally, in a handover completion state, only the termination (TDM/1) is released, and the termination (RTP/1) is continuously used as an anchor.